Demand for portable electronic devices is increasing each year. Example portable electronic devices include: laptop computers, personal data assistants (PDAs), cellular telephones, and electronic pagers. Portable electronic devices place high importance on total weight, size, and battery life for the devices.
Most portable electronic devices employ rechargeable batteries. Commonly used rechargeable batteries include Nickel-Cadmium (NiCad), Nickel-Metal-Hydride (NiMHi), Lithium-Ion (Li-Ion), and Lithium-Polymer based technologies. Charger circuits are commonly available for each of these types of battery technologies. Each charger circuit includes a shunt regulator to control the amount of charge that is delivered to the battery.
An example shunt regulator charging system is shown in FIG. 1. As shown in FIG. 1, the shunt regulator charging system includes a power source (PS), a shunt regulator (102), and a rechargeable battery (BATT). The power source (PS) includes a voltage source (VS) and a source resistance (RS). The shunt regulator (102) includes a NMOS transistor (MN), a PMOS transistor (MP), an amplifier (AMP), and three resistors (R1–R3).
In operation the power source provides a charging current (I) to the lithium battery through source resistance RS, PMOS transistor MP, and resistor R3. Resistor R3 converts the charging current (I) into a voltage (VSNS), which is used by other circuitry (not shown) to control the activation of transistor MP. PMOS transistor MP is activated during normal charging operations. Resistors R1 and R2 form a voltage divider that provide a feedback signal to amplifier AMP. Amplifier AMP compares the feedback signal to a reference voltage (VREF) and provides a control signal to transistor MN. Transistor MN, amplifier AMP, and resistors R1–R2 together operate as a shunt regulator that regulates the input voltage (VIN). The shunt regulator provides safe charging of lithium battery BATT by limiting the charging voltage (input voltage) similar to a zener diode.